Many gas turbine power plants comprise a number of combustors, wherein each combustor has multiple burning zones, such as a pilot section and a main burn section, which is adjacent to and downstream of the pilot section. For example, gas turbines of the W 501D5 type, manufactured by Westinghouse Electric Corporation, employ 14 combustors axially mounted about the longitudinal axis of the gas turbine, each combustor comprising a pilot section and a main burn section.
Regarding the W 501D5 gas turbine, during turbine start-up conditions, natural gas fuel is injected separately into both the pilot section and main burn section of each combustor. Electrical igniters are provided in the pilot section of two of the fourteen combustors. When these electrical igniters are fired, a flame is created in the pilot section, as the gas injected into that pilot section mixes with the surrounding air and burns. This flame is referred to as a diffusion flame since mixing of the fuel with the air occurs via a diffusion process. Hot combustion products from the lit pilot section flow into the associated main burn section, causing the natural gas fuel injected into that section to combust and burn.
In order to create a diffusion flame in the pilot sections of the remaining non-lit combustors, cross flame tubes are provided which connect the pilot sections of two adjacent combustors. As a result of the combustion process taking place in the pilot sections of the two electrically lit combustors, the pressure in these pilot sections increases above the pressure level in the pilot section of the neighboring, non-lit combustor. This pressure differential forces hot combustion gases to flow through the cross flame tubes and into the pilot section of the neighboring combustor. As these hot gases enter the pilot section of the neighboring combustor, the natural gas ignites and creates a diffusion flame in the pilot section. Accordingly, hot gas from that pilot section flows downstream into the adjacent main burn section, igniting the natural gas fuel which is injected into that main burn section. This procedure continues until all neighboring combustors are lit. At this point, the pilot sections are said to operate in a diffusion burn mode and the main burn sections operate in a lean burn mode.
Some time after turbine start-up, after all of the combustors have been lit, the turbine reaches base load operating conditions. At this point it is undesirable to maintain operation of the combustors with the pilot section in the diffusion burn mode, as undesirable levels of nitrogen oxide emissions are known to be produced during this type of operation. High levels of NOx emissions are produced when the pilot section operates in the diffusion burn mode since the injected natural gas fuel burns at a relatively high temperature.
Alternatively, it is known that, during operation of the turbine at base load conditions, NOx emission levels are reduced when the pilot section operates in a premixed mode. During premix mode operation the natural gas fuel is injected into the pilot section where it mixes intimately with inlet air in the pilot section without combustion. After mixing in the pilot section, the natural gas fuel-air mixture flows into the main burn section where it contacts the hot gases there and combusts and burns. Since the natural gas fuel-air mixture burns at a relatively low temperature, NOx emissions are reduced when compared with operation of the pilot section in the diffusion mode. This reduction in NOx emission levels may be as large as 40%.
Accordingly, to operate the turbine at base load conditions with the pilot section in a premix mode, it is necessary to extinguish the diffusion flame in the pilot section of each combustor, in order to shift from diffusion mode operation to premix mode operation. In order to extinguish the diffusion flame, the flow of the natural gas fuel into the pilot section is reduced until flame-out of the diffusion flame occurs. At this point, since natural gas fuel is independently injected into the main burn section, the main burn section continues to burn fuel there. Following flame-out of the diffusion flame, the flow of natural gas fuel into the pilot section is returned to operational levels and the natural gas fuel mixes with inlet air and flows into the main burn section at a low temperature. During this premix mode operation, the velocity of the natural gas fuel-air mixture in the pilot section is greater than the flame velocity of the flame generated by burning of the natural gas fuel in the main burn section. Therefore, flashback of the flame into the pilot section is prevented and a flame in the pilot section will not be present during premix mode operation.
During periods where it is necessary to operate the turbine at other than base load conditions, the turbine is unloaded by decreasing the flow of natural gas fuel injected into the main burn section to the point of flame-out in that section. However, prior to unloading it is necessary to reestablish a diffusion flame in the pilot section. This is necessary in order to maintain stable operation such that, when it is time to return the turbine to base load operation, there is still a flame in each combustor.
As stated previously, prior to flame-out in the main burn section, a diffusion flame cannot be reestablished in the pilot section using the flame in the main burn section, since the velocity of the natural gas fuel-air mixture in the pilot section is greater than the flame velocity. Also, auto-ignition of the natural gas fuel in the pilot section during premix mode operation is not a possible method of reestablishing a diffusion flame, as the temperature of the inlet air, which is approximately 700.degree. F. (371.degree. C.), is less than the auto-ignition temperature of natural gas fuels, which is greater than 1000.degree. F. (538.degree. C.).
The likelihood of successfully utilizing the two electrical igniters in conjunction with the cross flame tubes, as done during turbine start-up, to reestablish the diffusion flame in the pilot combustor is low. During premix mode operation, where the pressure in each combustor is approximately the same, the feasibility of cross-igniting due to a pressure differential across the cross flame tubes is speculative and therefore unreliable.
In the alternative, the diffusion flame could be reestablished by placing a non-retractable electrical igniter in the pilot section of each combustor. However, this presents a reliability problem in that the igniter may fail to spark. Also, for those gas turbine power plants which employ a number of combustors, the overall operational cost, as well as the chance of failed ignition, increases.
Therefore, there is a need for a system for reestablishing a diffusion flame in the pilot section of a combustor of a gas turbine power plant following operation of the pilot section in a premix mode where natural gas fuel mixes with air without combustion. The present invention provides a system which satisfies this need.